This invention relates to a communication system and to a receiving method. More particularly, the invention relates to a communication system and to a receiving method in which when an encoded signal could not be decoded correctly on a receiving side, the signal is retransmitted from the transmitting side. More specifically, the invention relates to a receiving method in a communication system that sends and receives informational data, the method having an ARQ (Automatic Repeat Request) function and an FEC (Forward Error Correction) function in order to improve data transmission quality (this method is referred to as a “hybrid ARQ”, or “H-ARQ”, method), and to a receiving apparatus and transmitting apparatus that implement the H-ARQ method.
(1) H-ARQ
In H-ARQ, a transmitter transmits as a packet all or part of an informational data block that has undergone error-correcting encoding. A receiver receives the packet and, if the received data cannot be decoded correctly, requests the transmitter to retransmit the data. In response to the retransmit request, the transmitter retransmits as a packet all or part of the same encoded data block in accordance with a stipulated method. In the sending and receiving of a systematic code in which the informational data per se (referred to as “systematic bits”) is contained in the code itself, the receiver can perform decoding using only the systematic bits even if there are no parity bits. Partial transmission, therefore, is an effective method.
FIG. 26 illustrates an example of a process for retransmitting a systematic code having an encoding rate of 1/3 (the systematic code is composed of systematic bits and parity bits having twice the size of the systematic bits). In this example, the data size of a packet capable of being transmitted at one time is the same as the informational data block itself, whereas non-redundant parity bits are transmitted twice to completely transmit all data. FIG. 27 is a block diagram of a transmitter 1 and receiver 2 in a communication system of such a systematic code. The transmitter 1 includes an error-detecting encoder 1a for appending an error-detecting code to informational bits to be transmitted; an error-correcting encoder 1b for applying error-correcting encoding processing to the informational bits to which the error-detecting code has been appended and for outputting the result; a transmission-pattern designating unit 1c for segmenting an informational data block in accordance with a stipulated method to thereby packetize the data, and for transmitting the packets successively; a rate matching unit 1d for executing rate-matching processing (e.g., punctured encoding); a modulator 1e of a modem for mapping signal points (e.g., for performing QPSK or 16QAM modulation; and a transmitting unit (not shown) for frequency-converting the modulated signal to a radio-frequency signal, applying high-frequency amplification and transmitting the amplified signal. As will be described later, the receiver 2 determines whether the information in a received packet has been decoded correctly or not and notifies the transmitter 1 of success/failure (ACK/NACK) of reception. A receiving unit (not shown) in the transmitter 1 frequency-converts a radio signal received from the receiver 2 to a baseband signal, a demodulator 1f of the modem demodulates the receive signal and an ACK/NACK discriminator 1g discriminates ACK/NACK. An H-ARQ controller 1h instructs the transmission-pattern designating unit 1c to perform retransmission if NACK is discriminated and to transmit a new information block if ACK is discriminated. If NACK is discriminated, the transmission-pattern designating unit 1c performs control in accordance with a stipulated method to retransmit transmit information that been stored in a buffer (not shown). If ACK is discriminated, then the transmission-pattern designating unit 1c deletes the transmit information that has been stored in the buffer and, packetizes a new information block and transmits the same.
The receiver 2 has a receiving unit (not shown) for frequency-converting the radio signal from the transmitter 1 to a baseband signal; a demodulator 2a of a modem for applying QPSK demodulation and outputting the demodulated data as likelihood (soft-decision) data; an average-value calculation unit 2b for calculating the average value of the amplitudes of the soft-decision demodulated data and deciding the quantization range and quantization levels; a quantizer 2c for quantizing the receive signal based upon the quantization levels and inputting the quantized signal to a de-rate matching unit 2d, the latter executing processing (e.g., punctured decoding) that is the reverse of rate matching; an H-ARQ buffer 2f for storing packet data that failed to be decoded the last time data was received; and an H-ARQ combiner 2e for combining data that has been stored in the H-ARQ buffer 2f and retransmission data just received. H-ARQ combining is performed by appropriately diversity-combining (as by maximal-ratio combining) the values of corresponding bits of the stored data and newly received data. In the event that only one value exists for corresponding bits, the value of this bit is used as is as the combined output. The receiver 2 further includes a post-combination average-value calculation unit 2g for calculating the average value of the combined signal and deciding the quantization range and quantization levels based upon the average value; a post-combination quantizer 2h for quantizing the combined signal based upon the quantization levels and inputting the quantized signal to an error-correcting decoder 2i, the latter executing error-correcting decoding processing and outputting decoded bits if decoding can be performed correctly; and an error detector 2j for detecting whether or not an error is present based upon the error-detecting code and outputting the result of detection.
The receiver 2 further includes a NACK/ACK discriminator 2k for instructing a notification-information encoder 2m whether to transmit NACK or ACK (abnormal reception or normal reception) to the transmitter 1 based upon the result of error detection; the notification-information encoder 2m for encoding notification information as instructed; and a modulator 2n for modulating the NACK/ACK notification information and transmitting the modulated signal to the transmitter 1 via a transmitting unit (not shown).
If decoding can be performed correctly, the data that has been stored in the H-ARQ buffer 2f is cleared from the buffer. If decoding cannot be performed correctly, then the stored data in the H-ARQ buffer 2f is updated by the H-ARQ combined signal. Further, in order for the H-ARQ combiner 2e to perform combining correctly, it is necessary to know the data structure of the packet transmitted. For example, in a case where a plurality of data formats are used to transmit data and the transmitter 1 transmits data adaptively in a prescribed format depending upon the propagation environment, it must be ascertained in which format the transmission was made. In such case the transmitter 1 gives such notification to the receiver 2 in parallel with the format information using a signal separate from the packet by means not shown.
(2) Quantization
In a case where decoding processing is implemented by a digital circuit, the receive data is quantized at a stipulated amplitude range and stipulated number of bits by the time it enters the decoder. The parameter that stipulates quantization is decided so as to obtain a performance equivalent to that which prevails when data that is not quantized is decoded by an ideal decoder. From the standpoint of circuit mounting, however, the number of bits is selected to be as small as possible in order to reduce the scale of the circuitry. With H-ARQ, it is expected that the performance obtained at retransmission will be equivalent to that obtained when combined data is decoded by an ideal decoder.
Conventionally, newly received data and stored data that are to undergo combining are quantized independently by stipulated parameters. These items of data are expressed by amplitude levels (e.g., the levels of the output data of demodulator 2a) that serve as a common platform in order to handle the data as one set of data, combining processing is executed, quantization processing is carried out again by a stipulated method and the resultant data is adopted as the input data to the decoder 2i. FIG. 28 illustrates an example of a case where uniform quantization is performed using two bits and four levels with the levels being spaced apart equally. For the sake of simplicity, all of the data is limited to positive data. In accordance with the retransmission patterns of FIG. 26, initially data having small reception amplitudes and composed solely of systematic bits is received, as indicated at (A) in FIG. 28. At the first retransmission, data composed only of parity bits of large amplitudes is received, as indicated at (B) in FIG. 28. Since the systematic bits and parity bits are quantized independently, the respective levels are expressed by quantization ranges and quantization codes at a stage prior to combination, the quantization code of the systematic bits is expressed as (3, 2, 4, 3, 1, 2, 2, 3) and the quantization code of the parity bits is expressed as (3, 4, 1, 4, 2, 3, 2, 1). In order to obtain common quantization levels at the time of combining, the level average (the average of 16 bits) of all data bits of the systematic bits and parity bits is calculated, a new quantization range (e.g., twice the average value) is decided from the calculated average, as shown at (C), and four new quantization levels L1 to L4 are set. In the case of FIG. 28, there is no combining processing because there is no overlapping of data. The data bits, therefore, are converted to code at the new quantization levels. If the maximum value at the new level is exceeded, then the maximum value is applied. As a result, the systematic bits become (0, 0, 1, 0, 0, 0, 0, 0), the parity bits become (4, 4, 2, 4, 4, 4, 4, 2), and this becomes the data input to the decoder 2i. 
(3) Deciding Combination of Feedback, which is Result of Decoding, and Transmit Data
Illustrated in FIG. 29 by way of example are the format of encoded information on a data channel in a case where H-ARQ stipulated by 3GPP Release 5 is employed and the format of information (referred to as “notification information” below) reported from the receiver 2 to the transmitter 1 in order to request retransmission. The notification information is composed of an ACK/NACK signal indicating success/failure of decoding, and CQI bits which are signals corresponding to the SN ratio of the receive data.
FIG. 30 illustrates the flow of a sequence up to transmission of the next transmit bit following transmission of notification information from the receiver 2 to the transmitter 1. Processing relating to the CQI signal corresponding to the SN ratio is omitted.
A base station constituting the transmitter 1 decides the transmission pattern, segments the encoded information blocks of FIG. 29 to packetize the same and transmits the packets (blocks) (steps 1001, 1002).
If a mobile station constituting the receiver 1 receives a packet (an encoded informational data block), then the mobile station executes de-rate matching processing, H-ARQ combining processing, decoding processing and ACK/NACK discrimination processing (steps 2001 to 2005), creates notification information and transmits the notification information to the transmitter 1 (step 2006).
The transmitter 1 receives the signal sent from the receiver, decodes the notification information from the receive signal and discriminates ACK/NACK from the notification information (steps 1003 to 1005). If the notification is ACK, the transmitter 1 encodes the next informational data block (step 1006), refers to the CQI bits and decides the bit size and modulation scheme of the new data block (i.e., decides the initial transmission pattern at step 1007). If the notification is found to be NACK at step 1005, on the other hand, then the transmitter 1 performs retransmission scheduling (step 1008) and selects the combination of bits to be transmitted (decides the retransmission pattern at step 1009). It should be noted that the retransmission pattern is decided in accordance with a stipulated method that depends upon the number of retransmissions.
Step 1007 or 1009 is followed by execution of rate matching processing and transmission of the encoded informational data block (steps 1010, 1011). The receiver 2 receives the data block, repeats the foregoing processing (step 2007) and executes sending and receiving of packets.
(4) Adaptive Encoding Error Control Scheme
An adaptive encoding error control scheme for changing the error-correcting encoding scheme or the encoding rate in accordance with the state of the transmission path has been proposed heretofore (see JP8-265304A). This example of the prior art uses an error controller on the receiving side to execute decoding according to all encoding schemes or encoding rates that might possibly be used on the transmitting side, apply error detection to each of the results of decoding and adopt a decoded result for which error could not be detected as the correct result of reception. Furthermore, the encoding scheme or encoding rate is selected on the receiving and is indicated to the transmitting side as feedback.
(1) First Problem
In the quantization method according to the prior art, a quantization method based upon the smallest number of quantization bits best suited in a case where application is to the receive data alone is also applied as is to quantization of the data that is the result of combining. However, such a quantization method is not necessarily the optimum quantization method. This will be described with reference again to the example of FIG. 28. In the series of transmission processes regarding the systematic code in the example of FIG. 28, the description will be in accordance with the retransmission patterns of FIG. 26.
In H-ARQ control, the average reception power of packets in a transmission process performed with regard to one set of data blocks is not necessarily constant. There are cases where transmission power is changed adaptively by the scheduling of the transmitter and cases where even though transmission power is constant, the reception power fluctuates owing to the occurrence of fading in the receive signal when the receiver (mobile station) moves. Accordingly, if systematic bits capable of being decoded themselves independently are selected preferentially as the data packet to be transmitted the first time (see FIG. 26), then the power of this packet will diminish, as indicated at (A) in FIG. 28, and the SN ratio will worsen. Consequently, decoding fails and the packet of the first retransmission is sent upon elapse of a fixed period of time. This packet is decided in accordance with a stipulated pattern in such a manner that parity bits are selected preferentially. However, the packet only has a small size that cannot contain all of the code bits and therefore only some of the parity bits can be transmitted. The reception power of this packet is very large, as indicated at (B) in FIG. 28, combination is performed at a large SN ratio and it can be expected that the result of combining the signal with the data of the systematic bits of the first transmission and performing decoding will be ACK with a high probability. It is assumed that ACK is actually obtained with an ideal decoder.
However, when the conventional quantization method is applied, as shown in FIG. 28, the distribution range of data amplitudes is too large with the systematic bits of (A) and the parity bits of (B), and the values of the data represented by systematic bits of small amplitude are rounded off almost to zero, as indicated at (C). Effective information is lost as a result.
(2) Second Problem
As indicated at (A) in FIG. 31, a problem arises also in a case where large-amplitude systematic-bit priority data is received in a second retransmission that follows FIG. 28. That is, when large systematic bits indicated at (A) of FIG. 31 are received as data of a second retransmission, new quantization levels L1 to L4 obtained when this retransmission data is combined take on levels near the original quantization levels of the parity bits of the first retransmission, as indicated at (B) in FIG. 31. However, at the time of combination in the first retransmission, parity-bit data of large amplitude that exceeds the maximum value is replaced by the maximum value prevailing at this time, as indicated by the dashed lines at (C) of FIG. 28. Consequently, the parity bits after the combination of the first retransmission become small in amplitude in comparison with the original data and information is lost. In other words, the reliability of the parity bits is lost and the probability that decoding will fail rises.
(3) Third Problem
With the conventional method of feeding result of decoding (ACK/NACK and encoding scheme or encoding rate) from the receiver to the transmitter, the result of combining in the receiver is not reported and therefore a set of retransmission data decided by stipulated scheduling not related to the result of combining is sent. This retransmission data is not necessarily the optimum data. For example, if a set of retransmission data is decided so as to retransmit a different combination of the parity bits from the second retransmission onward when error is detected because the values of data of the systematic bits have been rounded off almost to zero, then the process will continue advancing without the presence of the information of the systematic bits and, hence, the error will continue.